This application claims the benefit under 35 USC 119 of earlier filed EP Pat. App. 00204808.0, filed on Dec. 22, 2000.
Not Applicable
Not Applicable
The invention relates to a device for the inspection of patterns on objects, which device is provided with:
a carrier unit for carrying the objects during the inspection,
for each object an inspection unit that includes at least one particle-optical column for scanning the pattern to be inspected on the associated object,
a comparison circuit for comparing the scan signals that are produced by the particle-optical column in a first inspection unit and by the particle-optical column in a second inspection unit,
which device is arranged for the simultaneous inspection of corresponding patterns on a plurality of objects.
A device of the kind set forth is known from U.S. Pat. No. 5,641,960.
In the semiconductor industry there is a need for equipment that is suitable for the inspection of patterns written on wafers, for example, for the detection of defects arising during the manufacturing process. Such defects may be the cause of malfunctioning of the manufactured integrated circuits (ICs) so that they have to be rejected. As the smallest critical dimensions of the ICs become smaller and smaller, it is necessary to have inspection equipment available that is still capable of suitably discriminating such small details. The contemporary dimensions of the order of magnitude of 200 nm of the smallest details necessitate a resolution of the inspection equipment of approximately 50 nm; it is generally expected that this resolution will have to be improved even further in the near future.
Optical equipment that operates in the field of the visible light is no longer adequate to achieve such a high resolution. The use of particle-optical equipment, notably scanning electron microscopes (SEMs), however, enables suitable observation of such small details.
The cited United States patent describes a device for the inspection of patterns on objects, said device being arranged to inspect circuits on semiconductor wafers. The known device is provided with a carrier unit with a carrier or stage for the wafers that rotates during the acquisition of the signals that are required for the inspection. For each wafer there is provided an inspection unit in the form of an electron optical SEM column that carries out the wafer inspection. The entire wafer surface can be accessed for inspection by the SEM columns in that the carrier rotates relative to the columns and in that during the inspection the columns are displaced in the radial direction relative to the axis of rotation of the carrier. Because the described known device is arranged for the simultaneous inspection of corresponding patterns on different wafers, the scan signals produced by the particle-optical columns can be compared by a comparison circuit, so that the existence of a defect can be decided upon when a difference between these signals is detected. This comparison can be performed in xe2x80x9creal timexe2x80x9d, so that it is not necessary to form and maintain very large data bases with which the signals have to be compared and that it is not necessary either to carry out time-consuming digital comparison operations between the instantaneous scan signals and the stored data.
Because the known device is provided with only one column for each wafer, inspection in this device can take place at a comparatively low feed-through rate only. This can be demonstrated as follows. As the details of the patterns to be inspected become smaller, the number of details to be inspected for each wafer increases, that is, by the square of the detail reduction (the wafer dimensions remaining the same). As a result, inspection of the entire wafer surface is dispensed with and only the areas within the patterns in which the smallest detail size occurs are inspected, that is, the so-called Care Areas. The fraction of the pattern in which such Care Areas are situated is known as the Care Area Fraction (CAF) that typically has a value of the order of magnitude of 1%. The feed-through rate during inspection can be considerably increased by inspecting only the CAF; however, in order to make wafer inspection in future keep pace with the production rate of the wafers (which is necessary for on-line inspection of the wafers), the CAF must be strongly reduced, so that parts of the patterns that are prone to defects would have to be skipped; of course, such a development is undesirable.
The inspection of the wafers in the known device is performed by making the wafer rotate relative to the inspecting column. During this rotational displacement the passing wafer is irradiated by an electron beam that is produced by the column and is stationary relative to the column during the execution of the inspection scan. The area of the wafer that is inspected per revolution of the carrier is thus shaped as a circular path; if a larger area is to be inspected, for example, an area having a rectangular shape, such an area will have to be composed from a number of adjoining circular paths. This means that it is still possible to select a number of care areas for inspection, but also that the surface area of the circular paths that are situated between the care areas must also be covered. Consequently, the scan time cannot be reduced by skipping such intermediate areas.
This method of scanning also has the drawback that the factor limiting the speed of rotation (for example, the processing speed of the electronic circuitry used for the data comparison) is based on the highest speed that occurs, that is, the speed of the areas at the outer periphery of the circle of rotation, that is, the location where the maximum radial position of the columns is situated. All areas to be inspected within this maximum circle of rotation, therefore, have a non-optimum speed, so that the device is overproportioned for the vast majority of the areas to be inspected.
It is an object of the invention to provide a solution to the problem of maintaining the feed-through rate during on-line inspection of semiconductor wafers with increasingly greater circuit densities. To this end, the device in accordance with the invention is characterized in that the particle-optical columns are arranged to carry out an x-y scan of the pattern on the objects by x-y deflection of the particle beam that is produced by the relevant particle-optical column, and that the carrier unit is arranged to realize a rectilinear translatory feed-through direction for the objects. In the device in accordance with the invention the objects to be inspected (the wafers) are arranged underneath the inspection column in such a manner that they are oriented on the carrier in the direction perpendicular to the feed-through direction of the carrier (for example, to be referred to hereinafter as the x direction) in such a manner that they occupy the desired position relative to the column. In the (rectilinear translatory) feed-through direction (for example, referred to as the y direction) wafers are arranged in the desired location underneath the inspection column in such a manner that they occupy the desired position relative to the column in said direction as a result of adjustment of the feed-through distance. The scanning of the desired care areas takes place after such positioning in that the electron beam that is produced by the column is scanned across the area to be inspected in the customary manner. Because this scan is executed under the control of an electron beam, such scanning requires a substantially smaller amount of time than the physical rotary displacement of the objects relative to the column.
An embodiment of the device in accordance with the invention is arranged to superpose on at least one of the scan signals a periodic signal that varies linearly in time and whose period is greater than that of the relevant scan signal. During the scanning of an area to be inspected by means of a SEM, the electron beam is deflected in two mutually perpendicular directions, that is, the x direction and the y direction. This deflection is realized by applying periodic deflection signals that vary in time to the SEM. When a further linear signal is superposed on one or on both scan signals, not only the scan motion for the inspection is realized but also a linear motion of the entire inspection area. As a result of this step, the object to be inspected can be displaced relative to the column during the scan, so that the scan area can be made to move along with the moving object. The objects can thus be fed through very gradually and without abrupt changes; this may be an advantage especially in the case of wafer inspection.
A preferred embodiment of the device in accordance with the invention is constructed in such a manner that each of the inspection units is provided with an array of at least two particle-optical columns and that the particle-optical columns occupy a fixed position relative to one another.
Because each inspection unit includes a plurality of particle-optical columns, a higher feed-through rate can in principle be achieved. However, such a higher rate is possible only if corresponding columns within different inspection units can simultaneously inspect corresponding areas of different wafers. Generally speaking, the distance between two corresponding parts of the pattern (that is, areas of a similar structure within different patterns) on a wafer will not be the same as the distance between two corresponding columns. In that case real-time pattern comparison will be impossible. It could be deliberated to adapt the pitch of the position of the columns to the pitch of the CAFs to be inspected (that is, to make the positions of the columns within an inspection unit variable), but the device would then become much more complex than in the case of a fixed relative position of the columns.
Within the limits that are imposed by the amount of space available, however, an arbitrarily large number of particle-optical columns can be chosen for each inspection unit, that is, for each wafer, so that a gain in feed-through rate can still be obtained as will be illustrated on the basis of the following numerical example. By way of example it is assumed that the CAF amounts to 1%, that seven columns (1 to 7) are provided for each inspection unit, and that three wafers are inspected simultaneously; this means that there are three inspection units (A, B, C). The wafers to be inspected can be supplied on the carrier unit in such a manner and the columns can be arranged at such a distance from one another that a corresponding area of each wafer is inspected. Each time one column of each inspection unit is then in operation, for example, the columns A1, B1 and C1. In this example, therefore, three out of 21 columns are simultaneously in operation. However, it is now possible to increase the CAF considerably by choosing the areas to be inspected to be such that also the columns A2, B2 and C2 are simultaneously in operation, that is, also simultaneously with A1, B1 and C1. In the latter example six out of 21 columns are then simultaneously in operation. The same is also possible for the other columns, so that even more columns are simultaneously in operation. The feed-through rate can thus be increased without it being necessary to reduce the CAF, or the CAF can be increased without reducing the feed-through rate.
Because the columns do not move at the time of execution of the inspection scan, there will be no uncertainty as regards their position relative to one another and the limit of the resolution that can be achieved during the inspection will be determined only by the resolution of the columns themselves; in accordance with the present state of the art, this resolution may be of the order of magnitude of a few nanometers.
A further preferred embodiment of the device in accordance with the invention is provided with at least three inspection units. This embodiment offers the advantage that during the comparison of the scan signals a simple majority decision can be taken so as to decide which of the three signals is deviant, that is, to decide in which wafer the defect occurs.
The particle-optical columns in a further embodiment of the device in accordance with the invention columns are arranged along one line and the carrier unit is arranged to realize a feed-through direction for the objects that extends perpendicularly to the direction of said line. Each point of each wafer can thus be readily reached by the columns for inspection and the positioning accuracy of the wafers in the feed-through direction is maximum in the case of a rectilinear feed-through. When the columns are arranged along one line, the wafers are also arranged along one line, so that the positioning accuracy of the wafers is maximum in the line direction.
The particle-optical columns in another embodiment yet of the device in accordance with the invention are constructed as electron optical columns and the optical elements in the electron optical columns are constructed as electrostatic elements. For miniaturization of a SEM it is advantageous to utilize electrostatic optical elements, such as the beam deflection electrodes or the objective, because they can be constructed so as to be smaller than magnetic elements. This is due to the absence of the need for cooling means (notably cooling ducts for the lens coil), and due to the fact that the magnetic (iron) circuit of the lens must have a given minimum volume in order to prevent magnetic saturation. Moreover, because of the requirements that are imposed nowadays in respect of high vacuum in the sample space, electrostatic electrodes (being constructed as smooth metal surfaces) are more attractive than the surfaces of a magnetic lens since the surfaces thereof are often provided with coils, wires and/or vacuum rings.